Abstract
The reproductive effort as a function of the reproducing individual's weight and number of fetuses has been estimated. This estimate is based on models for energy requirements of reproducing females of the common vole Microtus arvalis (Pall.). This function forms the basis for construction of age (or weight) specific demographic fitness sets. Based on these, optimal reproductive rates are predicted for stable and cyclic populations. The predictions thus deduced find support in available field information. For instance, the model explains the larger litters and earlier age of maturation in cyclic microtine populations as compared to stable populations; further, it explains the increased litter size with increasing parity. On the basis of our analysis a new pattern is revealed; the maximal realized litter size is reached earlier in fluctuating populations than in more stable populations; thus, the ratio between litter size of old and young females is predicted to be largest in the most cyclic populations. This compares favourably with field data on several species.
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